152 



THE PHYSIOLOGY OF PLANTS 



reaching down to the old wood (ah). The reader must re- 

 member that he is looking at a transverse section of the 

 branch. At the time the injury took place, the cambium layer 

 {camh) of the branch was in contact with the old wood (all) ; on 

 the outside of the cambium was the old bast (r). After the 

 cut had been made, those cells of the bast and cortex which 

 were still capable of division, as well as the cambium cells, 

 grew out into the gap and formed the callus layers (c to c). 



K A CLT IN THE OUTER LAYERS OF A BRANCH OF Tilia europcea. 



Very soon the callus formed a protective cork layer {k and k), 

 and the latter was continued backwards to the cork layer (kk), 

 which had been produced by the cortex cells immediately be- 

 low the original cork, and which thus separated the injured, 

 and consequently dying, bast fibres from the living tissues. 

 In this way a continuous protective layer was formed round 

 the injured tissues, and the cambium layer (camb) started 

 forming new wood on the inside. This new wood (7ih) was 



